It is known that a bearing structure used for a high-speed rotating machine such as a supercharger includes a floating-bush type sliding bearing (floating-bush bearing). In the bearing structure, the sliding bearing (floating-bush) is rotatably disposed between a rotating shaft and a housing, and lubricating oil is supplied to a gap between the shaft and the bearing and to a gap between the bearing and the housing from corresponding oil supply sources.
In addition, compressing air or an air mixture to be supplied to a cylinder of an internal combustion engine in advance is referred to as “supercharge”, and a compressor used for the compression is referred to as a “supercharger”. Specifically, a supercharger driven by a gas turbine using exhaust gas of an engine is referred to as an exhaust gas turbine supercharger, or abbreviated to a turbocharger. Hereinafter, except for special cases, the turbocharger is simply called the supercharger.
In the bearing structure with the floating-bush bearing, the shaft rotates inside the bearing with an oil film therebetween, and the rotation of the shaft is accompanied by the rotation of the bearing. In addition, due to the oil film provided on inner and outer peripheral surfaces of the bearing, damping effects can be obtained. Since the difference between velocities of the shaft and the bearing is suppressed by the rotation of the bearing, energy loss caused by the shear stress in the lubricating oil is relatively small.
The floating-bush bearing has a problem in that unstable vibration sometimes occurs in a shaft and bearing system as a vibration mode. It is thought that the unstable vibration occurs due to a behavior of a viscous fluid in the gap between the shaft and the bearing, shaking of the shaft center, or the like, unlike mode characteristics (rotation primary vibration and the like) caused by mechanical unbalance, so that the unstable vibration includes self-excited vibration called oil-whirl, oil-whip, and the like (for example, see Patent Document 1).
In addition, since unstable vibration causes noise during high-speed driving, various techniques for suppressing the unstable vibration have been proposed (for example, see Patent Documents 2, 3, and 4). However, it is difficult to completely suppress the unstable vibration.
Patent Document 1: Japanese Unexamined Patent Application, first publication No. 2002-213450 (paragraph number 0043-0046, FIG. 10)
Patent Document 2: Japanese Unexamined Patent Application, first publication No. 2002-138846
Patent Document 3: Japanese Unexamined Patent Application, first publication No. 2002-213248
Patent Document 4: Japanese Unexamined Patent Application, first publication No. 2001-12460
Patent Document 5: Japanese Unexamined Patent Application, first publication No. H11-336744